The invention relates to a semiconductor device having a semiconductor body of a first conductivity type which is provided with a planar surface and in which a field effect transistor is formed which is composed of a number of parallel-connected subtransistors which each comprise a polygonal box-shaped cell of the semiconductor body, which cells each have a semiconductor zone of the second opposite conductivity type which is embedded in the semiconductor body and is provided with an edge directed at right angles to the surface and extending so that a peripheral part of the semiconductor body surrounding the first zone adjoins the surface. A second zone of the first conductivity type is embedded in this first zone, and the second zone adjoins the surface and is provided with a central opening and with an edge which extends at a substantially fixed distance from the edge of the first zone so that a central part and a narrow edge strip of the first zone adjoin the surface, the edge strip extending at least over part of its length at a substantially fixed distance from corresponding parts of edge strips of adjacent cells. The peripheral part of the semiconductor body surrounding the first zone serves as a drain zone of a subtransistor, while the second zone adjoining the surface serves as a source zone of the subtransistor. The narrow edge strip of the first zone located between the drain zone constituted by the peripheral part of the semiconductor body and the source zone constituted by the second zone serves as the channel zone of the subtransistor. The p-n junction constituted by the central part of the first zone and the part of the second zone surrounding this zone is shortcircuited by a metallization provided on the surface.
Such a semiconductor device may be used, for example, in high-power circuit arrangements. The field effect transistor present in the semiconductor device can be controlled directly from micro-electronic integrated circuits.
British Patent Application No. 2,033,658 discloses a semiconductor device of the kind mentioned above, in which the parallel-connected subtransistors each comprise a hexagonal cell of the semiconductor body. The narrow edge strip of the first zone, which constitutes the channel zone of a subtransistor, extends at a fixed distance from the edge of the cell and consequently also has a hexagonal shape. Another prior art hexagonally configured field effect transistor device is shown in French Pat. No. 2,438,917.
With comparatively low voltages between the source and the drain zones, such a transistor behaves like a resistor. The value of this resistor depends upon the size of the cross-section of the part of the semiconductor body through which current flows when the transistor is switched on. This part is constituted by the peripheral part of the semiconductor body surrounding the first zone. Its cross-section has a surface area which is approximately equal to the product of the length l of the narrow edge strip of the first zone and half the distance d between the edge strips of adjacent cells extending at a substantially fixed relative distance. It is of importance that the aforementioned resistor is as small as possible. Therefore, the surface area of the peripheral region should form the largest possible part of the overall cell surface area S. Since the distance d cannot be chosen freely--it depends inter alia upon tolerances which have to be taken into account during the manufacture of the semiconductor device--the ratio 1/S should be as large as possible. It has been found that for a hexagonal cell form, such as the known described semiconductor device, the ratio L/S is at most d.sup.-1. This is the case if 1=2.multidot.d.sqroot.3 and S=2.multidot.d.sup.2 .multidot..sqroot.3. The ratio 1/S can be considered as a filling factor, which for square and triangular cells also proves to be at most d.sup.-1.
In the known described transistor, a central part of the first zone adjoins the surface of the semiconductor body. This means that l and S for reasons of manufacturing technique have to be approximately 10.multidot.d and 15.multidot.d.sup.2, respectively, for d=8 .mu.m. The filling factor could be 125 mm.sup.-1, but for the reasons mentioned above the latter is about 30% lower.